The present invention relates generally to optical instruments and, more particularly, to a method and apparatus for measuring wavefront aberrations. The present invention is particularly useful, but not exclusively so, for measuring the optical wavefront in ophthalmic applications, e.g., measurement of aberrations of the eye, developing corrective devices such as lenses (e.g., contact, spectacle, and intraocular), and for evaluating the ocular aberrations before, during and after refractive surgery to improve vision.
The human eye is an optical system employing several lens elements to focus light rays representing images onto the retina within the eye. The sharpness of the images produced on the retina is a factor in determining the visual acuity of the eye. Imperfections within the lens and other components and material within the eye, however, may cause the light rays to deviate from a desired path. These deviations, referred to as aberrations, result in blurred images and decreased visual acuity. Hence, a method and apparatus for measuring aberrations is desirable to aid in the correction of such problems.
One method of detecting aberrations introduced by an eye involves determining the aberrations of light rays exiting from within the eye. A beam of light directed into the eye as a point on the retina is reflected or scattered back out of the eye as a wavefront, with the wavefront containing aberrations introduced by the eye. By determining the propagation direction of discrete portions (i.e., samples) of the wavefront, the aberrations introduced by the eye can be determined and corrected. In this type of system, increased accuracy in determining the aberrations can be achieved by reducing the size of the samples.
A general illustration of the generation of a wavefront is shown in FIG. 1. FIG. 1 is a schematic view of a wavefront 10 generated by reflecting a laser beam 12 off of the retina 14 of an eye 16. The laser beam 12 focuses to a small spot 18 on the retina 14. The retina 14, acting as a diffuse reflector, reflects the laser beam 12, resulting in the point source wavefront 10. Ideally, the wavefront 10 would be represented by a spherical or planar wavefront 20. However, aberrations introduced by the eye 16 as the wavefront 10 passes out of the eye 16 result in an imperfect wavefront, as illustrated by the wavefront 10. The wavefront 10 represents aberrations which lead to defocus, astigmatism, spherical aberrations, coma, and other irregularities. Measuring and correcting these aberrations allow the eye 16 to approach its full potential, i.e., the limits of visual resolution.
FIG. 2 is an illustration of a prior art apparatus for measuring the wavefront 10 as illustrated in FIG. 1. By measuring the aberrations, corrective lens can be produced and/or corrective procedures performed to improve vision. In FIG. 2, a laser 22 generates the laser beam 12 which is routed to the eye 16 by a beam splitter 24. The laser beam 12 forms a spot 18 on the retina 14 of the eye 16. The retina 14 reflects the light from the spot 18 to create a point source wavefront 10 which becomes aberrated as it passes through the lens and other components and material within the eye 16. The wavefront 10 then passes through the beam splitter 24 toward a wavefront sensor 26.
Typical prior art wavefront sensors 26 include either an aberroscope 28 and an imaging plane 30, as illustrated in FIG. 3, or a Hartman-Shack sensor 32 and an imaging plane 30, as illustrated in FIG. 4. The wavefront sensor 26 samples the wavefront 10 by passing the wavefront 10 through the aberroscope 28 or the Hartman-Shack sensor 32, resulting in the wavefront 10 producing an array of spots on the imaging plane 30. Each spot on the imaging plane 30 represents a portion of the wavefront 10, with smaller portions enabling the aberrations to be determined with greater accuracy. Generally, the imaging plane 30 is a charge coupled device (CCD) camera. By comparing the array of spots produced on the imaging plane 30 by the wavefront 10 with a reference array of spots corresponding to the wavefront of an ideal eye, the aberrations introduced by the eye 16 can be computed.
An example of a Hartman-Shack system described in U.S. Pat. No. 6,095,651 to Williams et al., entitled Method and Apparatus for Improving Vision and the Resolution of Retinal Images, filed on Jul. 2, 1999, is incorporated herein by reference.
The resolution of the aberrations in such prior art devices, however, is limited by the sub-aperture spacing 34 and the sub-aperture size 36 in an aberroscope 28 (see FIG. 3), and by the lenslet sub-aperture spacing 38, and focal length, in a Hartman-Shack sensor 32 (see FIG. 4). In addition, since each area is represented by a single spot, the amount of information captured for each area is limited. Also, because of foldover, reductions to sub-aperture spacing 34 and size 36 and lenslet sub-aperture spacing 38, the capabilities to obtain more detailed information are limited.
Foldover occurs in an aberroscope sensor 28, for example, when two or more spots 40A, 40B, and 40C on imaging plane 30 overlap, thereby leading to confusion between adjacent sub-aperture spots. Similarly, foldover occurs in Hartman-Shack sensors 32 when two or more spots 42A, 42B, 42C, and 42D on imaging plane 30 overlap. Foldover may result from a sub-aperture spacing 34, sub-aperture size 36, or lenslet spacing 38 which is too small; a high degree of aberration; or a combination of these conditions. Hence, the sub-aperture spacing 34 and sub-aperture size 36 in the aberroscope 28, and the lenslet sub-aperture spacing 38, and focal length in the Hartman-Shack sensor 32 must be selected to achieve good spatial resolution while enabling the measurement of large aberrations. Accordingly, the ability to measure a high degree of aberration comes at the expense of spatial resolution and/or dynamic range and vice versa.
The constraints imposed by the aberroscope and Hartman-Shack approaches limit the effectiveness of these systems for measuring aberrations with a high degree of accuracy. These limitations prevent optical systems from achieving their full potential. Accordingly, ophthalmic devices and methods which can measure aberrations with a high degree of accuracy would be useful.
The present invention provides for an apparatus and method for determining the aberrations of a wavefront with a high degree of accuracy. The apparatus includes a beam splitter for separating the wavefront into two components, mirror arrays for focusing each of the components to a plurality of discrete lines with the discrete lines of one component having a different orientation than the discrete lines of the other component, and an imaging device for detecting the discrete lines to determine wavefront aberrations. The method includes separating the wavefront into two components, focusing each of the components into a plurality of discrete lines with the discrete lines of one component having a different orientation than the discrete lines of the other component, and detecting information related to the discrete lines.
By generating discrete lines which represent the wavefront, the apparatus and method of the present invention are capable of measuring the wavefront with a high degree of accuracy. Since each of the plurality of discrete lines have a different orientation, the plurality of discrete lines essentially represent the wavefront as a grid. The present invention is able to provide more accurate information than prior art systems since the grid lines of the present invention provide more information for each section of the grid than the spots which would be generated by prior art systems to represent equivalent areas.
In a system for measuring the wavefront of an eye, the wavefront originates as a point source within the eye. The point source is generated by directing a beam of radiation (e.g., a laser) into the eye and scattering or reflecting the beam. A beam splitter disposed in the path of the laser beam directs the laser beam into the eye. The retina of the eye functions as a diffuse reflector for reflecting or scattering the beam. The wavefront resulting from the point source passes out of the eye and through the beam splitter to the wavefront sensor of the present invention. The wavefront sensor measures the aberrations of the wavefront introduced by the eye. Aberrations are then computed by a processor coupled to the wavefront sensor.